Conventionally, a vehicle-mounted ultrasonic sensor vibrates a diaphragm made of materials, such as aluminum, which do not allow light to pass therethrough, by using a ceramic resonator or the like, thereby transmitting/receiving ultrasonic waves through the diaphragm toward the surroundings of the vehicle. An obstacle detection apparatus detects an obstacle in the area surrounding the vehicle on the basis of a result of the transmission and reception of the ultrasonic wave that returns after being transmitted by the ultrasonic sensor and reflected by the obstacle. When detecting an obstacle in the area surrounding the vehicle by using the ultrasonic sensor, the obstacle detection apparatus urges the driver to pay attention to the obstacle by, for example, outputting a buzzer sound from a speaker or blinking a warning light, in the vehicle cabin.
Further, in an obstacle detection apparatus for vehicles described in Patent Literature 1, for example, a transmission and reception unit and a light emitting diode (LED) are contained in a bezel for fixing an ultrasonic sensor to a bumper. This bezel is made of an optically transparent material, and projects light emitted by an LED included therein to the outside of the vehicle, via a portion disposed around an optically non-transparent transmission and reception unit, which is a diaphragm made of aluminum. By projecting light emitted by the LED from the bezel surrounding the diaphragm made of aluminum to the outside of the vehicle when an obstacle is detected, it is possible to inform the outside of the vehicle that the vehicle is approaching.